At present, manually shifted automotive transmissions operate primarily on the principle of forced synchronization. The shifting action establishes a positive connection between the main transmission shaft and the respective gear wheel. The positive connection between the elements is ultimately attained by means of a gearshifting or sliding sleeve, aided by various synchronization elements.
Gearshifting or sliding sleeves carry a gear-tooth system of complex design on the inside surface of the sleeve. As is well known, the synchronization process involves first bringing the inside gear teeth of the sliding sleeve in contact with the locking gear of a synchronization ring, and then, as synchronization proceeds, pushing them through the locking gear into a claw gearing of the gear wheel. This ends the gear shifting process. In order to ensure that the gearing as described will not disconnect, particularly under load changes in a particular gear, the gearing of the inside sleeve surface, among others, is also fitted with undercut tooth flank sections which, when viewed along the axis of the sleeve, taper down toward the inside in the direction of the plane of symmetry.
As a normal practice in the design of automotive transmissions, a gearshifting sleeve is used to engage two gears on one shaft by means of shifting it either to the left or to the right, so that for this reason alone it is logical to design the gearshifting sleeve symmetrically with respect to a central plane at right angles with the sleeve axis. Minor differences in design between the left and right half-sleeves usually occur when, for instance, a gearshifting sleeve is used to engage one forward and one reverse gear.
Synchronization in shifting transmissions along with the design elements and techniques used for this purpose are described in more detail in, for example, the article "Synchronization, an Important Design Element of our Shifting Transmissions" by Dipl. Ing. E. Brugel in the German periodical "ZF Ring" (1988), issue 1.
Due to the design requirements mentioned above, gearshifting sleeves can so far only be produced from a ring-shaped blank by means of very involved and expensive machining methods. As an alternative, the special gear-tooth profiles are manufactured by an initial machining operation in a preliminary stage, followed by undercut rolling.
The manufacture of a gearshifting or sliding sleeve represents a significant cost factor in producing a transmission. Consequently, continuing efforts have been made to reduce manufacturing costs for this unit. Some of these efforts have culminated in patents. However, it has so far not been possible to abandon the method of expensive inside machining for producing the special gear-tooth system in gearshifting or sliding sleeves.